Quantum-Dot Windows Harvest Solar Energy

PORTLAND, Ore. — Quantum dots are already revolutionizing displays, such as those used in widely praised Kindle Fire e-readers whose backlight uses a quantum-dot enhanced film (QDEF) manufactured by Nanosys. Now researchers are poised to revolutionize solar energy collectors with quantum dots.

By harvesting light coming from the sun with embedded quantum dots, the researchers hope to turn windows into efficient solar-panel concentrators. Their strategy is to place photovoltaic (PV) solar cells around the edges of quantum-dot-impregnated windows, thus turning them into luminescent solar concentrators (LSCs). Los Alamos National Laboratory, in cooperation with Italy's University of Milano-Bicocca (UNIMIB), recently demonstrated optical efficiencies for such LSC windows of greater than 10 percent and an effective concentration factor of more than four.

"Our device is a light-harvester -- a concentrator that captures light from a large area and directs it to a much smaller PV cell," Victor Klimov, lead researcher on the project at the Center for Advanced Solar Photophysics (CASP) at Los Alamos National Laboratory, told EETimes.

For the proof-of-concept demonstration, Klimov's team embedded the quantum dots into a transparent plastic material with PV solar cells around its edges, with the help of colleagues at UNIMIB, including Sergio Brovelli, who worked at Los Alamos National Labs until 2012 but who is now a faculty member at UNIMIB.

"The quantum dots re-emit absorbed solar light at a longer wavelength, which then propagates in the regime of total internal reflection towards the PV cell installed at the edge of the LSC device," says Klimov.

Quantum dots are highly efficient emitters, demonstrating emission efficiencies approaching 100 percent, but previous attempts to use them in LSCs of practical dimensions were not successful. The problem was that the quantum dots reabsorbed many of the re-emitted photons that were intended to be harvested by the edge-mounted PV cells. To solve that problem, Klimov and colleagues engineered quantum dots that shifted the wavelength of the re-emitted photons using an approach of Stokes-shift-engineering, named after the 19th century Irish physicist George Stokes.

A giant Stokes-shift was engineered into the quantum dots by combining two different materials, cadmium selenide (CdSe) and cadmium sulfide (CdS), in a core-shell geometry. A small CdSe core served as an emitter while a thick CdS shell played the role of a light-harvesting antenna. Since CdS has a wider bandgap than CdSe, the light re-emitted by the CdSe core exhibited a large low-energy shift with respect to the onset of strong optical absorption defined by the CdS shell. This strategy resulted in a giant Stokes shift, which helped eliminate losses to re-absorption.

What would be the application of the Quantum dot enabled solar energy harvesting? The mobile devices would be able to harvest solar energy to generate power sufficient for charging the battery of the mobile devices? Recently while driving to my office I kept my phone on the seat next to mine. Sunlight was falling on its display and I was thinking (or rather wishing) that, there should have been a technology such that the display of the mobile device (or a transparent screen on it...the screen protector itself) to be able to harvest electrical energy from the solar energy to charge the battery of the device, so that my mobile will get charged on the way to my office... :)

Your idea is very innovative, since it turns the display into a solar energy concentrator. The inventors of the quantum-dot window are concentrating on scaling them up so that windows in homes and buildings can also be concentrated solar energy harvesters. Scaling down to mobile device size would require photovoltaic cells that were very thin to fit around the edge of a display, but the concentration factor should be impressive since the entire face of the display would be funneling incident light to the PVs. Only time will tell if your idea is workable, but I see promise.

While visiting one of the North Eastern states of India where Buddhism is prominant, I bought a toy that has a wheel similar to the one you find in Budhist monastries , which are turned by hand by the devotees.

The toy wheel I bought has a small array of solar panels . I have kept it on the dashboard of my car . whenever sunlight falls on it while driving, the wheel starts rotating by the energy harvested by the solar panels.

I think this idea can be expanded to develop a solar energy harvestor for in car mobile charging.

the novely of the idea is to engineer the quantum dots so that they remit absorbed solar light for PV panels. If the costs can be brought down and such panels can be installed in the window pans then it will bring down the electricity costs and scarcity by some level.

There are two big problems with combing a phone display with a solar cell.

1: Standard office light is ~1000 times weaker than full sunshine. Our eyes are actually amazing in that they can handle nearly 10 orders of magnitudes of photon intensity.

2: The solar cell will block outgoing light as well, implying that you'd have to crank up the power on the display in order to achieve the same brightness.

These two issues combined make it almost impossible to make an energy profit with a display-side solar cell. The back side is feasible, but you'd have to leave the phone face down. I do that anyway for privacy reasons.

@Ogemaniac: Thank you for raising those points. I agree with what you have said. But my thought came while I was driving to office...for outdoor application. It takes about one hour for travelling to office and mostly in India, we get abundant sunlight in almost all seasons. I did not think about indoor application and I agree that display will have to be brighter, which would consume more power...but there could be a solution thought out. I like your idea of putting it on the back side!